Four-wave mixing in slow light photonic crystal waveguides with very high group index

This work was supported by the EPSRC - UK Silicon Photonics consortium.We report efficient four-wave mixing in dispersion engineered slow light silicon photonic crystal waveguides with a flat band group index of n(g) = 60. Using only 15 mW continuous wave coupled input power, we observe a conversion efficiency of -28 dB. This efficiency represents a 30 dB enhancement compared to a silicon nanowire of the same length. At higher powers, thermal redshifting due to linear absorption was found to detune the slow light regime preventing the expected improvement in efficiency. We then overcome this thermal limitation by using oxide-clad waveguides, which we demonstrate for group indices of n(g) = 30. Higher group indices may be achieved with oxide clad-waveguides, and we predict conversion efficiencies approaching -10 dB, which is equivalent to that already achieved in silicon nanowires but for a 50x shorter length.Publisher PDFPeer reviewe

Ultracompact and low-power optical switch based on silicon photonic crystals

Switching light is one of the most fundamental functions of an optical circuit. As such, optical switches are a major research topic in photonics, and many types of switches have been realized. Most optical switches operate by imposing a phase shift between two sections of the device to direct light from one port to another, or to switch it on and off, the major constraint being that typical refractive index changes are very small. Conventional solutions address this issue by making long devices, thus increasing the footprint, or by using resonant enhancement, thus reducing the bandwidth. We present a slow-light-enhanced optical switch that is 36 times shorter than a conventional device for the same refractive index change and has a switching length of 5.2 m.The work was funded through the EU FP6-FET “Splash” project and we acknowledge the Nanostructuring Platform of EU FP6-NoE “epixnet” for technical support. T. P. White is supported by an 1851 Royal Commission Research Fellowship

Flexible metamaterials at visible wavelengths

We report on the fabrication and characterization of plasmonic structures on flexible substrates (Metaflex) and demonstrate the optical properties of a single layer of Metaflex. The layer exhibits a plasmonic resonance in the visible region around 620 nm. We show experimental and numerical results for both nano-antennas and fishnet geometries. We anticipate the use of Metaflex as a building block for flexible metamaterials in the visible range.Publisher PDFPeer reviewe

Photonic microstructures as laser mirrors

Deeply etched 1-D third-order Bragg reflectors have been used as mirrors for broad-area semiconductor lasers operating at 975-nm wavelength. From a threshold and efficiency analysis, we determine the mirror reflectivity to be approximately 95%. The design of the GaAs-based laser structure features three InGaAs quantum wells placed close (0.5 μm) to the surface in order to reduce the required etch depth and facilitate high-quality etching. Despite the shallow design and the proximity of the guided mode to the metal contact, the threshold current density (J_(th) = 220 A/cm^2 for infinite cavity length) and internal loss (α_i = 9±1 cm^(−1)) are very low

Random lasing in uniform perovskite thin films

Following the very promising results obtained by the solar cell community, metal halide perovskite materials are increasingly attracting the attention of other optoelectronics researchers, especially for light emission applications. Lasing with both engineered and self-assembled resonator structures, such as microcrystal networks, has now been successfully observed, with the low cost and the simple solution-based process being a particular attraction. The ultimate in simplicity, however, would be to observe lasing from a continuous thin film, which has not been reported yet. Here, we show random lasing action from such a simple perovskite layer. Our lasers work at room temperature; they are deposited on unpatterned glass substrates and they exhibit a minimum threshold value of 10 µJ/cm2. By carefully controlling the solution processing conditions, we can determine whether random lasing occurs or not, using identical precursors. A rather special feature is that some of the films exhibit single and dual mode lasing action, which is rarely observed in random lasers. Our work fully exploits the simplicity of the solution-based process and thereby adds an important capability into the emerging field of perovskite-based light emitters

Searching for Dark Matter at the LHC with a Mono-Z

We investigate a mono-Z process as a potential dark matter search strategy at the LHC. In this channel a single Z boson recoils against missing transverse momentum, attributed to dark matter particles, $\chi$, which escape the detector. This search strategy is related, and complementary to, monojet and monophoton searches. For illustrative purposes we consider the process $q\bar{q} -> \chi\chi Z$ in a toy dark matter model, where the Z boson is emitted from either the initial state quarks, or from the internal propagator. Among the signatures of this process will be a pair of muons with high pT that reconstruct to the invariant mass of the Z, and large amounts of missing transverse energy. Being a purely electroweak signal, QCD and other Standard Model backgrounds are relatively easily removed with modest selection cuts. We compare the signal to Standard Model backgrounds and demonstrate that, even for conservative cuts, there exist regions of parameter space where the signal may be clearly visible above background in future LHC data, allowing either new discovery potential or the possibility of supplementing information about the dark sector beyond that available from other observable channels.Comment: 11 pages, 13 figure

Ultracompact (3 μm) silicon slow-light optical modulator

This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (FOM), which is financially supported by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO). The work is also supported by the research programs NanonextNL and MEMPHIS, funded by the Dutch ministry of economic affairs. We also acknowledge financial support by the EPSRC through the “UK Silicon Photonics” grant.Wavelength-scale optical modulators are essential building blocks for future on-chip optical interconnects. Any modulator design is a trade-off between bandwidth, size and fabrication complexity, size being particularly important as it determines capacitance and actuation energy. Here, we demonstrate an interesting alternative that is only 3 mm long, only uses silicon on insulator (SOI) material and accommodates several nanometres of optical bandwidth at 1550 nm. The device is based on a photonic crystal waveguide: by combining the refractive index shift with slow-light enhanced absorption induced by free-carrier injection, we achieve an operation bandwidth that significantly exceeds the shift of the bandedge. We compare a 3 mm and an 80 mm long modulator and surprisingly, the shorter device outperforms the longer one. Despite its small size, the device achieves an optical bandwidth as broad as 7 nm for an extinction ratio of 10 dB, and modulation times ranging between 500 ps and 100 ps.Peer reviewe

Photonic crystals for light-emitting devices

Photonic crystals or photonic bandgap (PBG) structures promise to revolutionize optoelectronics by making anew class of highly efficient, low noise light emitters possible. We present data to show that their properties, in particular 2D systems, have now been fully characterized in the relevant semiconductor material system and at near-IR wavelengths, so effort can be redirected towards making active light emitters. As a first example, we present a semiconductor laser with one output mirror designed according to PBG principles. From threshold and efficiency data, we derive a reflectivity of 95 +/- 10 percent for this mirror, which underlines the viability of the PBG approach for practical devices. In order to realize the full potential of photonic crystal light emitters, however, important material issues need to be considered. Non- radiative recombination, for example, is a big problem when the photonic crystal is an integral part of the active region because of the relatively large areas of exposed surface. Several possible solutions to this problem are presented